Aircraft: EMB ERJ 145 ER/LR
Phase of flight: Cruise
Anomaly: Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor

Cause of smoke is unknown to me at this time. I was the pilot monitoring [when] the flight attendant called up to the front and noted that she saw a smoke/ haze in the back of the cabin. She later indicated to us that several of the passengers had noticed the smoke and had also notified her. I asked her to come to the front of the aircraft and call me when she had done so. My FO utilized his phone to take a picture through the peephole so we could see what was occurring. We noted the smoke in the rear of the cabin. I began to smell the acrid odor in the flight deck. At that point I commanded masks on and began QRH procedures. I told them that [we intended to divert to a nearby airport]. I spoke with the FA again and told her to find the source possibly in the walls or ceiling. I did have several calls between myself and the FA in the descent. The FO got established on a visual approach. I elected to have him continue the pilot flying role so I could continue to monitor the emergency. Once we landed I brought the aircraft to a complete stop on the runway. I immediately called the FA and inquired if there was still smoke. The FA noted that she could still smell the smoke and I was concerned that we still had an active fire onboard. At that point I told the FA we were going to be evacuating on the runway. I made an announcement to the passengers to evacuate out the left and right side of the aircraft and to go to the nose of the aircraft. I completed the emergency evacuation QRH and noted that the FO was unable to egress out of the flight deck door. I decided that we needed someone on the ground to assist the evacuation at this point. I egressed through the Captain DV window and helped several of the passengers egress through the main cabin door as that was where the majority of the passengers were. Once we were completely egressed I began coordinating.Since cause of smoke is unknown at this time I cannot state; however I have a few lessons I would like to pass on.1. After the cabin O2 masks were deployed the mask door above the entrance caused difficulty in opening the flight deck door. This was unexpected and slowed egress.2. Locating the lead or incident commander is vital. I was the First Officer and PF. While en route the Flight Attendant contacted the cockpit to notify us of a smoke/haze that was in the back of the aircraft in which she was made aware of from a passenger ringing their call button. Since the FA informed us that she was in the rear of the aircraft; the Captain asked her to continue to the forward cabin and contact us again to see if there was any noticeable difference throughout the cabin. While the FA was still on the phone with the flight crew; I decided to use my iPhone camera to peer through the cockpit peephole to see if I could see any of the smoke. As the Captain and I zoomed in on the photo that I took; we quickly noticed that we could not even see the FA in the rear of the aircraft through the smoke developing in the cabin. It was then that we started to notice the haze that was developing in the cockpit and the acrid smell coinciding with it and we quickly donned our oxygen masks. Center gave us [a nearby alternate] as an option and we took it and began an expedited descent. There were 2-3 additional calls between the cabin crew and cockpit as we continued our descent and once again after landing; where a decision was made to do an evacuation on the runway to eliminate any potential of being onboard in case a fire was developing. The Captain and Flight Attendant made it clear that the passengers were to deplane in the forward [part] of the cabin through the entrance door; but some of the people in the rear took it upon themselves to remove the emergency exit doors and evacuate over the wings. EMB-145 flight crew reported diverting to an alternate airport and evacuating on the runway after experiencing smoke in the cabin enroute.